The human eye has
special cells in the retina, which can sense red, green, and blue light.
Every color that we can see is made up of some combination of these three
"primary" colors. For example, yellow is a combination of red
and green. White is a combination of red, green, and blue. The same "RGB"
color system is used in cameras and computer screens. Since the camera
works like the human eye, it can capture every color that the human eye
can see. You can adjust the sliders below, to see how any color can be
made through various combinations of red, green, and blue.Since every
color is a combination of three components, color is said to have three
dimensions.

Typical cameras can
sense red, green, and blue light. Every color in a photo is made up of
some combination of these three "primary" colors. For example,
yellow is a combination of red and green. White is a combination of red,
green, and blue. This "RGB" color system mimics the human eye.
We have special cells in the retina of the eye, called "cones",
which detect red, green, or blue light. Since the camera works like the
human eye, it can capture every color that the human eye can see. You
can adjust the sliders below, to see how any color can be made through
various combinations of red, green, and blue.

Red

Green

Blue

Since every color
is a combination of three components, color is said to have three dimensions.
By adding the ultraviolet sensor, we create a four-color or tetrachromatic
imaging system. This mimics the function of the bird's eye, which can
perceive ultraviolet light, as well as red, green, and blue.

Each
of the retina cells, called cones, can actually detect light over a broad
range of wavelengths. The sensitivities overlap so that a pure yellow
light of 550 nm will activate both the red and the green cones. However,
a yellow object typically reflects a broad range wavelengths, covering
the red and green part of the spectrum. The diagram below shows the wavelength
senstivities of the three types of cones in the human eye.

Wavelength sensitivities of the blue, green,
and red cones in the human eye.

Wavelength sensitivities of the four
types of cones in the eye of a bird.

Birds have the same
three types of cones in the retina. But they also have a fourth type,
which detects ultraviolet light. (Not all birds can see ultraviolet. In
some birds, the fourth cone detects violet light instead.) For birds,
color has four dimensions. They can see a multitude of different colors
that we humans cannot see or even imagine.

The special camera system that I am using has an ultraviolet image sensor
in addition to the RGB sensor of a typical digital camera.
By adding the ultraviolet sensor, we create a four-color or tetrachromatic
imaging system. This mimics the function of the bird's eye, which can
perceive ultraviolet light, as well as red, green, and blue.

Converting for the Human Eye

One of the challenges
of my work has been to find the best way to present the four-dimensional
color information to the human eye. Inevitably, some information is lost
in the process, as we simply cannot perceive four dimensions of color.
Here is a summary of the methods I have tried, with their advantages and
disadvantages.

stereogram

color overlay

texture overlay

reassignment

Stereogram. Originally I planned to present the images as stereograms,
where one eye would take in the RGB channels, and the other eye would
take in the ultraviolet. In principle, this method would allow the human
to see four color channels at the same time. In practice, the human visual
system cannot interpret color information from the paired images.

Color Overlay. Another
thought was to overlay a purple color (or some other color) onto the image
to represent ultraviolet. It's possible to produce some neat effects this
way, but it's misleading, because ultraviolet isn't really purple. Also,
the overlay doesn't combine with other colors in a natural way. In this
example, the tail feathers should appear white, not UV-colored.

Texture Overlay.
Finally, I devised a way to show the four dimensions of the avian
color system. Our eyes can see only three dimensions of color, but we
can perceive a fourth dimension, which is texture. By overlaying a textured
pattern onto the image, I was able to show the fourth dimension, the ultraviolet
light. This
is done without affecting the visible colors that we know and love. However,
the texture overlay can miss fine details like the narrow eye ring on
this bird.

Reassignment. This is the method of squeezing the image into a
smaller portion of the spectrum, which I have used in the photo gallery.
Although we lose the four-dimensional aspect of bird vision, this method
produces some beautiful images. Although familiar colors are shifted toward
the red end of the spectrum, the ultraviolet combines with visible light
in a very natural way to reveal new color combinations that were previously
hidden to our human eyes.